Lithium batteries abound in the portable electronic devices that are toted daily. However, the scarcity and price of both lithium and the other metals used to create batteries provides the impetus to move beyond lithium-based technology. By virtue of its position in group I of the periodic table, lithium shuttles only one electron at a time between the two electrodes. With this project, funded by the Solid State and Materials Chemistry Program in the Division of Materials Research at NSF, the researchers at the University of Michigan Ann Arbor develop materials for magnesium ion batteries. Magnesium, a group II metal, can shuttle two electrons at a time. In principle, doubling the number of electrons that react at the electrodes doubles the amount of charge that can be stored in the same volume of space. Storing more charge with the same-sized battery represents a significant advance in energy storage for portable electronics and potentially for electric vehicles. Magnesium also has the potential to yield safer batteries since this metal is less reactive with air and moisture than is lithium. However, developing this technology requires learning what chemical reactions occur at the solid electrode/ liquid electrolyte interfaces of a battery and how these reactions change the electrode surface over time, which is the focus of this project. The goal is ensuring that both safety and high charge-storing capacity are maintained over the entire battery lifetime. In addition to the advancing scientific understanding, this project involves graduate students, undergraduate students, and high school students from diverse backgrounds who collaborate to combine different forms of spectroscopy and microscopy in order to interrogate what magnesium molecules form in liquid solution and to observe how these molecules transform to move electrons through lab-scale batteries.
PART 2: TECHNICAL SUMMARY
This project, funded by the Solid State and Materials Chemistry Program in the Division of Materials Research at NSF, addresses two critical questions at the anode/electrolyte interface using Lewis-acid salts of magnesium alkoxide electrolytes: 1) what are the structure and composition of matter at the electrode/electrolyte interface? 2) what are the size, shape, and distribution of defects at the electrode? To answer the first question, the researchers employ electrochemistry using polished platinum single crystals, solution NMR spectroscopy, and Raman microscopy to probe the electrolyte/bulk solution interface. To answer the second question, they rely on electron microscopy and scanning electrochemical microscopy to probe surface reconstruction as well as macroscopic changes in the morphology of the solid electrode under varied conditions. Although the lithium-ion battery field has demanded answers to these questions to develop this familiar technology, the chemistry of magnesium is quite different. One difference that is a major advantage for magnesium is that its hexagonal crystal structure leads to metal deposits in platelet form, not as dangerous dendritic wires that could short-circuit the battery. That said, the relationship between the salt composition (what ions are present and at what concentration) and the electrodeposition growth mechanism is unknown in magnesium. The first aspect of this research is composed of experiments to specify what ionic species reside at the electrochemical interface starting from the traditional Guoy-Chapman-Stern model of the electrochemical double layer (where excess charge accumulates adjacent to the electrode). With the second thrust, the researchers study how the atomic surface and the microstructure of magnesium and the underlying electrode change with continued cycling that includes potential adsorption/desorption of ions. Overall, this knowledge teaches the battery community to connect known electrolyte design principles to long-term high current (i.e.-high power) battery function.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
